1. Field of the Invention
This invention relates to the shaping of glass sheets and in particular to bending iron molds for forming reverse curvatures in hot glass sheets.
2a. Technical Considerations
Bent glass sheets are commonly used as glazing closures in vehicles such as automobiles and the like. For such applications the glass sheets must be bent to precisely defined curvatures dictated by the configuration and outline of the openings in the vehicles in which the bent sheets are to be installed as well as the overall vehicle styling. At the same time it is important that the bent sheets meet stringent optical requirements so that the viewing area of the resulting shaped window is free of optical defects that would interfere with good vision through the window. After forming, the windows may be heat strengthened to increase their resistance to damage resulting from impact.
The trend in automotive styling toward the use of glass closures that conform with the shape of the vehicle requires more complicated shapes which in turn require additional bending techniques. These new configurations include compound bends such as S-shaped curves, i.e., a reverse curvature along one direction of the glass sheet.
It is common practice in the glass sheet bending art to press bend the glass sheets supported in a horizontal position either on a series of conveyor rolls or by a gaseous hearth bed which supports the glass in close relation thereover by hot gas, lifting the glass from proximity to the support plane provided by either the conveyor rolls or the gaseous support bed into engagement with a vacuum holder and then depositing the glass onto a ring-like member which is interposed between the plane of support and the bottom surface of the vacuum holder.
In the heating and shaping of glass sheets, it is also common practice to use contoured bending molds that support a flat glass sheet and convey it through a heating lehr. As the temperature of the glass increases and approaches its heat softening temperature, it begins to sag under the force of gravity and conforms to the contours of the shaping rails on the mold. If required, the bending molds may include hinged end sections to help facilitate bending portions of the glass sheet to sharp curvatures that could not normally be obtained through normal sag bending techniques. Where windshield glass is being shaped, two glass sheets or doublets, may be conveyed through the lehr on the bending mold and shaped simultaneously.
Prior to this present invention, a reverse curvature in a midportion of a glass sheet on a bending mold conveyed through a heating lehr could not be made in a conventional sag bending operation because there was no way to sag the glass to a reverse configuration.
2b. Patents of Interest
U.S. Pat. No. 2,932,129 to Alexander et al. teaches the bending or a pair or glass sheet blanks on an outline bending mold and subsequent cutting of the bent glass sheets to shape.
U.S. Pat. No. 3,155,485 to Ritenour et al. teaches a bending mold shaping rail with non-marring inserts. In particular, a block of graphited carbon is located at the cut point of an articulating outline mold such that a rigid glass sheet is supported on the graphited blocks prior to heating.
U.S. Pat. Nos. 3,976,462 to Sitara and 4,375,978 to Reese et al. disclose bending iron molds used in sag bending operations. Each includes contoured shaping rails supported by posts on a main support frame that is conveyed through a heating lehr. Flat glass sheets are positioned on the shaping rail of the bending mold and sag into contact with the rails as the glass sheets are heated in the lehr.
U.S. Pat. No. 4,305,746 to Hagedorn et al. teaches a press bending arrangement including a ring type mold construction and a plurality of shaping pads mounted within the outline of the shaping ring. The heat softened glass sheet is conveyed into a shaping station wherein the glass sheet is preliminarily bent by contoured conveying rolls. The glass sheet is then stopped, vertically lifted off the rolls by the shaping ring and shaping pads and pressed against an upper shaping mold. The ring mold and shaping pads thereafter lower the shaped glass sheet back onto the conveyor rolls which continue to convey the glass sheet downstream to a cooling area.
U.S. Pat. No. 4,508,556 to Bennett et al. teaches a method and apparatus for bending glass sheets to complicated shapes including an S-shaped transverse bend. Flat glass sheets are heated in a furnace and conveyed into a shaping station wherein the glass is supported on a formed gas hearth support bed. A curved outline lifting mold is positioned around the gas hearth support bed and lifts the hot glass sheets off the bed and into contact with an upper vacuum mold to form the desired "S" configuration. The outline mold is then lowered and a shuttle ring is positioned beneath the held glass on the upper vacuum mold to receive the glass and remove it from the shaping station for further processing.
U.S. Pat. No. 4,606,749 to Nushi et al. teaches a method and apparatus for shaping glass wherein heat softened glass sheets are lifted off supporting conveyor rolls and pressed between a pair of vertically aligned press faces. The lower press face is slotted to receive the conveyor rolls when the press face is in a lowered position. A first ring mold positioned about the periphery of the lower press face initially lifts the glass off the rolls and a main ring mold positioned about the lower press face forms the periphery of the glass.